Heart disease is the leading cause of death in the United States. A heart attack, also known as an acute myocardial infarction (AMI), typically results from a blood clot or “thrombus” that obstructs blood flow in one or more coronary arteries. AMI is a common and life-threatening complication of coronary artery disease. Coronary ischemia is caused by an insufficiency of oxygen to the heart muscle. Ischemia is typically provoked by physical activity or other causes of increased heart rate when one or more of the coronary arteries is narrowed by atherosclerosis. AMI, which is typically the result of a completely blocked coronary artery, is the most extreme form of ischemia. Patients will often (but not always) become aware of chest discomfort, known as “angina”, when the heart muscle is experiencing ischemia. Those with coronary atherosclerosis are at higher risk for AMI if the plaque becomes further obstructed by thrombus.
There are a number of portable monitors that attempt to detect AMI. Monitors that include wearable sensors (e.g. a medical-vest with electrodes) may be somewhat inconvenient for patients. Chronically implanted sensors provide the possibility for continuous monitoring without many of the inconveniences associated with wearable monitors. One type of implantable monitor known as the Guardian (Angel Medical Systems, the assignee of the present invention), which is currently undergoing clinical trials in the United States, includes an electrode chronically implanted within the heart. An intracardiac electrode may provide a strong signal at the cost of requiring intracardiac implantation. Another type of implantable monitor can rely upon subcutaneous electrodes, which are less invasive, but receive smaller amplitude signals compared to intracardiac electrodes.
Regardless of the device, many parameters that are involved in heart monitoring are heart rate dependent. Handling heart rate dependence of parameters presents a difficult problem.
U.S. patent application Ser. No. 11/594,806, invented by Fischell et al., filed November, 2006, entitled “System for the Detection of Different Types of Cardiac Events” describes an implantable cardiac event monitor that detects acute ischemia based on ST segment shifts. The described device categorizes electrogram segments according to heart rate; different heart rate ranges (e.g. 60 beats/min.-90 beats/min. and 91 beats/min.-110 beats/min.) are associated with different thresholds for detecting ischemia. The '806 application also describes requiring ST segment changes to persist before acute ischemia is detected; the persistence criteria may be heart rate dependent. U.S. Pat. No. 6,609,023 to Fischell et al., issued August 2003, also describes persistence criteria.
U.S. patent application Ser. No. 11/898,673, invented by Hopenfeld and owned by the assignee of the present invention, filed September 2007, entitled “Waveform Feature Value Averaging System and Methods for the Detection of Cardiac Events”, describes cardiac event detection tests that involve computing the average change in a parameter over time. (E.g. Average ST segment change of 3%/minute.)
In the context of body surface electrocardiograms, it is known that ST changes persist after recovery from exercise (i.e. high heart rate). Indeed, the trajectory of the ST vs. heart rate curve after exercise has been proposed as a marker for coronary artery disease. (Kligfield P, Lauer M S. “Exercise electrocardiogram testing: beyond the ST segment.” Circulation. 2006 Nov. 7; 114(19):2070-82.) The slope of the ST/HR curve in the setting of exercise stress testing has also been proposed as a marker for ischemia. Id. It also known to use of the slope of the ST/HR curve as a marker for chronic ischemia. Kligfield P, Ameisen O, Okin P M. Heart rate adjustment of ST-segment depression for improved detection of coronary artery disease. Circulation. 1989; 79(2):245-55.
Dealing with heart rate changes poses difficulties. One possibility for handling this problem is described in U.S. patent application Ser. No. 11/710,902 to John et al., filed Feb. 27, 2007, entitled “Systems and methods of medical monitoring according to patient state. According to this application, parameter thresholds are adjusted after a heart rate change has occurred. An alternative is described in the '806 application, which involves detecting whether a heart rate has changed, and excluding data associated with heart rate changes from cardiac detection tests.
In U.S. Pat. No. 6,397,100 to Stadler and Shannon, ST segment values are low pass filtered to ensure that very rapid changes, which may be caused by axis shifts, are not considered to be ST shifts caused by ischemia. Two different low pass filters are applied, resulting in two different filtered signals. One filtered signal is representative of very slow ST band drift. The other filtered signal is representative of the true ST level excluding high frequency axis shift. ST segment deviation indicative of ischemia is equal to the difference between the filtered signals.
Despite all of the foregoing work that has been done, there is still a need for an effective subcutaneous or surface based system for monitoring ischemia.